The purpose of the proposed research is to provide a detailed morphological analysis of the development of the rabbit testis, from its earliest differentiation to the initiation of spermatogenesis. The study will consist of (1) electron microscopic observations of developing germ cells, Sertoli cells, interstitial cells and tubular limiting membranes at multiple stages of fetal and postnatal development, and (2) ultrastructural and biochemical analysis of the Golgi apparatus isolated from spermatocytes and spermatids during the early stages of acrosome formation in the postnatal period. Testicular specimens will be obtained from fetal and postnatal New Zealand white rabbits at closely spaced intervals, beginning with the onset of gonadal differentiation in the fetus and continuing through the initial spermatogenic cycles during early postnatal development. At the same time, blood samples will be collected for determination of LH, FSH and gonadal steroids (performed in the laboratory of Dr. Robert Jaffe) in order to correlate changes in circulating hormones with morphologic changes in the developing testis.